1. Field of the Invention
The invention relates to a method for the direct joining of metal pieces to oxide-ceramic by heating the ceramic substrates covered with metal pieces in an oxygen-containing atmosphere to a temperature above the eutetic temperature of the metal and the oxide, but below the melting point of the metal.
2. Description of the Prior Art
German Published Non-Prosecuted Application DE-OS No. 25 08 224 discloses a method of this type. In this method, preoxidized metal is used so as to make possible adhesion between the metal and the ceramic at all points as well as in joints with a large area. In addition, the metal piece is pre-bent so that it makes contact only gradually with ceramic substrate during the heating process. Thereby, possible gas bubbles between the metal and the ceramic are supposed to be pushed out to the outside. This method, however, is cumbersome and expensive; also, if the metal piece and/or the ceramic substrate are preformed in a complicated manner, bending of the metal piece is not always possible, or the metal piece does not follow the substrate sufficiently uniformly. In addition, the exposed metal surface is coated during the heating with an oxide layer which must later be removed, considering the reactive atmosphere of 0.01 to 0.5 volume-% oxygen (remainder nitrogen) that must be used.
German Published Non-Prosecuted Application DE-OS No. 26 33 869 describes a method in which a metal foil, preferably of copper, is first oxidized, whereupon the metal foil with the oxide layer is brought into contact with an oxide-ceramic substrate, preferably of aluminum or beryllium oxide. Thereupon, the metal foil and the substrate are heated in an inert atmosphere at a temperature between the eutectic temperature of copper and copper oxide and the melting temperature of copper until an undereutectic melt has been formed between copper and the substrate. Heating in an inert atmosphere, however, does not lead to the desired strongly adhering joint between copper and ceramic because, at the process temperatures indicated, CuO is reduced to Cu.sub.2 O, and Cu.sub.2 O is reduced to Cu unless a certain minimum partial oxygen pressure is maintained in the ambient atmosphere. Reduced Cu, however, does not form a bond with the ceramic.
German Published non-Prosecuted Application DE-OS No. 26 33 869, proposed to use an oxygen-containing copper material without oxide coating in lieu of preoxidized copper. Apart from the fact that here also, no bond between copper and ceramic can be produced unless a minimum partial oxygen pressure is maintained in the oven atmosphere, it has been found in tests that the oxygen dissolved in the metal diffuses to the contact surface between copper and ceramic and remains occluded there, which leads to the formation of gas bubbles and thereby to a defective bond between the metal and the ceramic.
From German Published Non-Prosecuted Application DE-OS No. 23 19 854, a further method for directly joining metals to non-metallic substrates is known which differs from the just-described method by the provision that a bare, non-preoxidized metal foil, particularly of copper, is to be joined to the oxide ceramic substrate in a reactive, particularly oxygen-containing atmosphere. In this method, the oxygen in the atmosphere must first oxidize the copper superficially before the bonding process proper can proceed. However, it has been found that, particularly in the case of joints with a large area between ceramic and copper, the oxygen cannot penetrate in sufficient quantity into the gap between the ceramic and the copper, so that bubble-like places without adhesion are produced. In addition, the copper foil is coated on the side facing away from the ceramic substrate with a thick black copper oxide layer if the amounts of oxygen of 0.01 to 0.5 volume-% are present as indicated. This copper oxide layer must be removed in an additional operation before further processing is possible, particularly for use in the semiconductor industry. To use an oxygen content of less than 0.01 volume-% is expressly discouraged since then no bond between the copper and the ceramic is supposed to be produced.
From British Pat. No. 761,045 a method is likewise known for the direct joining of copper to ceramic. There, a large number of process variants is described in which the oven atmosphere has at least at times, an oxidizing and at least at times a reducing pressure. Heating in a vacuum is also proposed. It is a common feature of all process variants that the joining of copper and ceramic is to take place in a temperature range of 1083.degree. C. to 1235.degree. C., i.e. above the melting temperature of copper and below the melting temperature of copper oxide. The formation of excessively thick oxide layers on the exposed copper surfaces is to be prevented here by masking with a protective material such as aluminum oxide or silver. The application and subsequent removal of the protective material, however, necessitates additional process steps. Furthermore, the necessary temperature range is very high in a disadvantageous manner.